1. Field of the Invention
The present invention relates to light emitting diodes (LED's) and is particularly directed to LED's which are operable at room temperature and which efficiently emit light upon excitation with low voltages.
2. Description of the Prior Art
LED's convert electrical energy into light energy by means of a radiative recombination of carriers (electrons and holes) across a rectifying junction in a semiconductor. Early LED's included III-V compound semiconductors in which light emission is produced as a result of a radiative recombination of carriers at a p-n junction. A basic limitation of these early LED's is that the energy band-gap of most III-V compound semiconductors is such that the resultant light emission is in the infrared wavelength region. Inefficient up-converters must then be used to convert the infrared into visible light.
II-VI compounds are known to exhibit efficient direct wide-band-gap transitions, and have therefore been extensively investigated for use in diodes which will emit shorter wavelength light. Unfortunately, the same wide band-gap which makes them desirable for such applications also mitigates against the production of p-n junctions in all but a few such compounds. The difficulty in forming a p-n junction has caused II-VI compound LED research to be directed to the treatment of II-VI compounds for enabling a direct band-gap radiative recombination in the II-VI compounds by non p-n junction techniques. An exemplary survey of non p-n junction radiative recombination mechanisms in II-VI compounds is set forth in an article by A. G. Fischer, Electroluminescence in II-VI Compounds, 541-599, at pages 572-578; contained within the book, "Luminescence of Inorganic Solids", P. G. Goldberg, editor, New York Academic Press, 1966, New York, New York.
In spite of such limitations, some degree of success in forming II-VI compound LED's is evidenced by U.S. Pat. No. 3,541,375 and U.S. Patent application Ser. No. 851,906, filed Aug. 21, 1969. Both references disclose that the II-VI compounds must be doped with extrinsic luminescent centers, since intrinsic radiative recombinations having an energy nearly equal to the band-gap were believed unlikely. Such LED's produce radiation having an energy characteristic of the luminescent center and are incapable of producing near band-gap emission having an energy nearly equal to that of the band-gap of the II-VI material.